Monolayer and multilayered thin films deposited on a substrate are used in a number of optical and other applications. The layers of such thin films are usually less than 1 micrometer, and sometimes much less than 1 micrometer, in thickness. They must be of precisely deposited thicknesses to be useful.
The thin films are typically deposited by a vacuum deposition process such as electron beam deposition. A source of the deposited material and the target substrate are placed into a vacuum chamber, and the source is operated to deposit the required material to the required thickness. There are typically multiple sources as required for the multiple layers.
The operation of the sources is calibrated so that the thickness of the deposited layers is reasonably well known. However, variations in the deposition rate usually are present so that the thicknesses of the layers cannot be known with sufficient precision, just from the deposition parameters.
Monitoring devices are therefore employed to measure the thickness of the thin film as it is gradually deposited, and to halt the deposition when the required thickness is reached. The monitoring device may measure directly on the test specimen of interest, or on a surrogate test specimen present solely for the purpose of providing the thickness measurement.
Monitoring devices typically operate by optical reflection of a light beam from, or optical transmission of a light beam through, the thin film being measured. In one such instrument, the Leybold OMS 3000, the light from a broadband tungsten halogen light source is split into dual beams. The two beams are each directed through an optical chopper. One of the chopped light beams is reflected from or transmitted through the thin film being measured, and then provided to a monochromator. The other chopped light beam serves as a reference and is provided to the monochromator. The intensity of the reflected or transmitted beam, after normalization for the reference light intensity, serves as a measure of the thickness of the thin film being deposited.
This apparatus is operable, but the present inventors have observed that it suffers from poor performance in its signal-to-noise ratio and an insufficient resolution bandwidth. The result is that its performance is limited in its function of providing a real-time measurement of the thickness of the thin film being deposited. There is therefore a need for an improved thickness measurement apparatus for monitoring the thin films as they are being deposited. The present invention fulfills this need, and further provides related advantages.